Semiconductor fabrication includes a series of processes that produce electrical circuits in a semiconductor, e.g., a silicon wafer, in accordance with a circuit design. These processes are carried out in a series of chambers. Successful operation of a modern semiconductor fabrication facility requires a steady stream of wafers to be moved from one chamber to another in the course of forming electrical circuits in the wafer.
One important process is plasma etching, which is a process of transferring a pattern in a layer of mask material into another layer under the mask, such as a layer of conductive or dielectric material, by removing the layered material from the wafer surface. Such process inevitably generates different kinds of etch by-products, such as silicon oxide and organic polymer, depending on the layered material and the etch chemistry. Some of the by-products deposit onto interior surfaces of the chamber in which the plasma etching process is performed.
The continuing build-up of by-products on interior surfaces, such as the chamber wall, presents two challenges to semiconductor fabrication. First, the structure of the accumulated by-products is not stable. Thus, by-products tend to peel off the chamber wall generating particles and flakes that can fall upon the wafer surface, causing product defects, such as a short circuit between two conductive lines or a discontinuity where the upper layer cannot cover the debris. Second, the by-products remaining on the chamber wall react with the plasma and deleteriously affect the etch performance, a phenomenon that is also referred to as “process drift”.
To mitigate the impact of etch by-products, chamber cleaning is required to periodically remove the deposition from the chamber wall. To do this, the chamber is taken out of production, and a cleaning plasma, such as a CF4+O2 plasma for cleaning silicon oxide deposited during silicon etching, is introduced into the chamber. This plasma reacts with the deposition and the products of this reaction are pumped out of the chamber. After such chamber cleaning, however, it has been observed that a clean chamber wall makes the chamber unsuitable for immediate production wafer etching. This is referred to as “first wafer effect”. Chamber seasoning is a procedure of etching a series of blank silicon wafers to restore a chamber wall condition that is suitable for production wafer etching. After chamber seasoning, a thin layer of silicon oxide covers the chamber wall. The chamber is then returned to production wafer etching until the next round of chamber cleaning and seasoning becomes necessary.
Some key factors for evaluating etch performance include etch rate, etch selectivity, and undercut. Etch rate refers to the rate at which a layered material is removed from a wafer surface. Etch selectivity is defined as the ratio of etch rates between two layers under the same conditions. Undercut is a measure of the lateral extent of the etch under the mask. The smaller the undercut, the better the etch profile control.
A critical issue in process monitoring and chamber seasoning is how to identify the process drift and when to stop seasoning to return to production wafer etching. The conventional practice in the art is to measure the change of etch rate by periodically loading monitor wafers into the chamber. Such practice, however, causes too many interruptions to the production or seasoning and lowers the chamber throughput. In addition, such approach is empirically based and tends to stop production prematurely for cleaning or result in over-seasoning.
Therefore, it is highly desirable to develop a method of process monitoring and chamber seasoning that does not rely on measuring the etch rate and thereby avoids interruptions to production or seasoning. It is also preferred that such method monitors the chamber wall condition in a manner so as to provide real-time, accurate information about process drift and chamber seasoning.